.\"	$NetBSD: dlfcn.3,v 1.27 2010/01/24 22:54:14 wiz Exp $
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.Dd January 18, 2010
.Dt DLFCN 3
.Os
.Sh NAME
.Nm dlopen ,
.Nm dlclose ,
.Nm dlsym ,
.Nm dladdr ,
.Nm dlctl ,
.Nm dlerror
.Nd dynamic link interface
.Sh LIBRARY
(These functions are not in a library.
They are included in every
dynamically linked program automatically.)
.Sh SYNOPSIS
.In dlfcn.h
.Ft "void *"
.Fn dlopen "const char *path" "int mode"
.Ft "int"
.Fn dlclose "void *handle"
.Ft "void *"
.Fn dlsym "void * restrict handle" "const char * restrict symbol"
.Ft "int"
.Fn dladdr "void * restrict addr" "Dl_info * restrict dli"
.Ft "int"
.Fn dlctl "void *handle" "int cmd" "void *data"
.Ft "char *"
.Fn dlerror "void"
.Sh DESCRIPTION
These functions provide an interface to the run-time linker
.Xr ld.so 1 .
They allow new shared objects to be loaded into the process' address space
under program control.
.Pp
The
.Fn dlopen
function takes the name of a shared object as the first argument.
The shared object is mapped into the address space, relocated, and
its external references are resolved in the same way as is done
with the implicitly loaded shared libraries at program startup.
.Pp
The
.Fa path
argument can be specified as either an absolute pathname to a shared object
or just the name of the shared object itself.
When an absolute pathname is specified,
only the path provided will be searched.
When just a shared object name is specified, the same search rules apply that are
used for
.Dq intrinsic
shared object searches.
.Pp
Shared libraries take the following form:
.Do lib Ns Ao name Ac Ns .so Ns Oo .xx Ns Oo .yy Oc Oc Dc .
.Pp
If the first argument is
.Dv NULL ,
.Fn dlopen
returns a
.Fa handle
on the global symbol object.
This object
provides access to all symbols from an ordered set of objects consisting
of the original program image and any dependencies loaded during startup.
.Pp
The
.Fa mode
parameter specifies symbol resolution time and symbol visibility.
One of the following values may be used to specify symbol resolution time:
.Bl -tag -width "RTLD_LAZYXX" -offset indent
.It Dv RTLD_NOW
Symbols are resolved immediately.
.It Dv RTLD_LAZY
Symbols are resolved when they are first referred to.
This is the default value if resolution time is unspecified.
.El
.Pp
One of the following values may be used to specify symbol visibility:
.Pp
.Bl -tag -width "RTLD_GLOBAL" -compact -offset indent
.It Dv RTLD_GLOBAL
The object's symbols and the symbols of its dependencies will be visible to
other objects.
.It Dv RTLD_LOCAL
The object's symbols and the symbols of its dependencies will not be visible to
other objects.
This is the default value if visibility is unspecified.
.El
.Pp
To specify both resolution time and visibility, bitwise inclusive OR one of
each of the above values together.
If an object was opened with
.Dv RTLD_LOCAL
and later opened with
.Dv RTLD_GLOBAL ,
then it is promoted to
.Dv RTLD_GLOBAL .
.Pp
.Fn dlopen
returns a
.Fa handle
to be used in calls to
.Fn dlclose ,
.Fn dlsym ,
and
.Fn dlctl .
If the named shared object has already
been loaded by a previous call to
.Fn dlopen
.Pq and not yet unloaded by Fn dlclose ,
a
.Fa handle
referring to the resident copy is returned.
.Pp
.Fn dlclose
unlinks and removes the object referred to by
.Fa handle
from the process address space.
If multiple calls to
.Fn dlopen
have been done on this object, or the object was one loaded at startup time,
or the object is a dependency of another object
then the object is removed when its reference count drops to zero.
.Fn dlclose
returns 0 on success and non-zero on failure.
.Pp
.Fn dlsym
looks for a definition of
.Fa symbol
in the shared object designated by
.Fa handle ,
and all shared objects that are listed as dependencies.
The symbol's address is returned.
If the symbol cannot be resolved,
.Dv NULL
is returned.
.Pp
.Fn dlsym
may also be called with special
.Fa handle
values.
.Fn dlsym
respects symbol visibility as specified by the
.Fn dlopen
.Fa mode
parameter.
However, the symbols of an object's dependencies are always visible to it.
All shared objects loaded at program startup are globally visible.
Only the symbols in the main executable that are referenced by a
shared object at link time will be visible unless it has been linked
with the --export-dynamic option where all of its symbols will be
visible.
The following special
.Fa handle
values may be used with
.Fn dlsym :
.Bl -tag -width "RTLD_DEFAULTXX" -offset indent
.It Dv NULL
Interpreted as a reference to the executable or shared object
from which the call is being made.
Thus an object can reference its own symbols and the symbols of its
dependencies without calling
.Fn dlopen .
.It Dv RTLD_DEFAULT
All the visible shared objects and the executable will be searched in the order they
were loaded.
.It Dv RTLD_NEXT
The search for
.Fa symbol
is limited to the visible shared objects which were loaded after the one issuing the
call to
.Fn dlsym .
Thus, if
.Fn dlsym
is called from the main program, all the visible shared libraries are searched.
If it is called from a shared library, all subsequently visible shared
libraries are searched.
.It Dv RTLD_SELF
The search for
.Fa symbol
is limited to the shared object issuing the call to
.Fn dlsym
and those shared objects which were loaded after it that are visible.
.El
.Pp
.Fn dladdr
examines all currently mapped shared objects for a symbol whose address --
as mapped in the process address space -- is closest to but not exceeding
the value passed in the first argument
.Fa addr .
The symbols of a shared object are only eligible if
.Va addr
is between the base address of the shared object and the value of the
symbol
.Dq _end
in the same shared object.
If no object for which this condition holds
true can be found,
.Fn dladdr
will return 0.
Otherwise, a non-zero value is returned and the
.Fa dli
argument will be used to provide information on the selected symbol
and the shared object it is contained in.
The
.Fa dli
argument points at a caller-provided
.Va Dl_info
structure defined as follows:
.Bd -literal -offset indent
typedef struct {
	const char  *dli_fname;     /* File defining the symbol */
	void	    *dli_fbase;     /* Base address */
	const char  *dli_sname;     /* Symbol name */
	const void  *dli_saddr;     /* Symbol address */
} Dl_info;
.Ed
.Pp
The structure members are further described as follows:
.Bl -tag -width "dli_fnameXX"
.It Li "dli_fname"
The pathname of the shared object containing the address
.Fa addr .
.It Li "dli_fbase"
The base address at which this shared object is loaded in the process
address space.
This may be zero if the symbol was found in the internally generated
.Dq copy
section
.Po
see
.Xr link 5
.Pc
which is not associated with a file.
.It Li "dli_sname"
points at the nul-terminated name of the selected symbol
.It Li "dli_saddr"
is the actual address
.Pq as it appears in the process address space
of the symbol.
.El
.Pp
Note: both strings pointed at by
.Va dli_fname
and
.Va dli_sname
reside in memory private to the run-time linker module and should not
be modified by the caller.
.Pp
In dynamically linked programs, the address of a global function will
point to its program linkage table entry, rather than to the entry
point of the function itself.
This causes most global functions to appear to be defined within the
main executable, rather than in the shared libraries where the actual
code resides.
.Pp
.Fn dlctl
provides an interface similar to
.Xr ioctl 2
to control several aspects of the run-time linker's operation.
This interface is
.Ud
.Pp
.Fn dlerror
returns a character string representing the most recent error that has
occurred while processing one of the other functions described here.
If no dynamic linking errors have occurred since the last invocation of
.Fn dlerror ,
.Fn dlerror
returns
.Dv NULL .
Thus, invoking
.Fn dlerror
a second time, immediately following a prior invocation, will result in
.Dv NULL
being returned.
.Sh SEE ALSO
.Xr ld 1 ,
.Xr rtld 1 ,
.Xr link 5
.Sh HISTORY
Some of the
.Nm dl*
functions first appeared in SunOS 4.